Secondary cell batteries are constructed using the various secondary
cells already described. The lead-acid battery is one of the most common batteries in use
today and will be used to explain battery construction. The nickel-cadmium battery is
being used with increasing frequency and will also be discussed.

Figure 2-9 shows the makeup of a lead-acid battery. The container
houses the separate cells. Most containers are hard rubber, plastic, or some other
material that is resistant to the electrolyte and mechanical shock and will withstand
extreme temperatures. The container (battery case) is vented through vent plugs to allow
the gases that form within the cells to escape. The plates in the battery are the cathodes
and anodes that were discussed earlier. In figure 2-10 the negative plate group is the
cathode of the individual cells and the positive plate group is the anode. As shown in the
figure, the plates are interlaced with a terminal attached to each plate group. The
terminals of the individual cells are connected together by link connectors as shown in
figure 2-9. The cells are connected in series in the battery and the positive terminal of
one end cell becomes the positive terminal of the battery. The negative terminal of the
opposite end cell becomes the negative terminal of the battery.

Figure 2-9. - Lead-acid battery construction.

Figure 2-10. - Lead-acid battery plate arrangement.

The terminals of a lead-acid battery are usually identified from one
another by their size and markings. The positive terminal, marked (+) is sometimes colored
red and is physically larger than the negative terminal, marked (-).

The individual cells of the lead-acid battery are not replaceable,
so in the event one cell fails the battery must be replaced.

The nickel-cadmium battery is similar in construction to the
lead-acid battery with the exception that it has individual cells which can be replaced.
The cell of the nicad battery is shown in figure 2-11.

Figure 2-11. - Nickel-cadmium cell.

The construction of secondary cell batteries is so similar, that it
is difficult to distinguish the type of battery by simply looking at it. The type of
battery must be known to properly check or recharge the battery. Each battery should have
a nameplate that gives a description of its type and electrical characteristics.

Q27.Other than the type of cell used, what is the major difference between the
construction of the lead-acid and nicad battery?
Q28.How is the type of battery most easily determined?

BATTERY MAINTENANCE

The following information concerns the maintenance of secondary-cell batteries and is
of a general nature. You must check the appropriate technical manuals for the specific
type of battery prior to performing maintenance on any battery.

Specific Gravity

For a battery to work properly, its electrolyte (water plus active
ingredient) must contain a certain amount of active ingredient. Since the active
ingredient is dissolved in the water, the amount of active ingredient cannot be measured
directly. An indirect way to determine whether or not the electrolyte contains the proper
amount of active ingredient is to measure the electrolyte's specific gravity. Specific
gravity is the ratio of the weight of a certain amount of a given substance compared to
the weight of the same amount of pure water. The specific gravity of pure water is 1.0.
Any substance that floats has a specific gravity less than 1.0. Any substance that sinks
has a specific gravity greater than 1.0.
The active ingredient in electrolyte (sulfuric acid, potassium hydroxide, etc.) is heavier
than water. Therefore, the electrolyte has a specific gravity greater than 1.0. The
acceptable range of specific gravity for a given battery is provided by the battery's
manufacturer. To measure a battery's specific gravity, use an instrument called a
HYDROMETER.

The Hydrometer

A hydrometer, shown in figure 2-12, is a glass syringe with a float
inside it. The float is a hollow glass tube sealed at both ends and weighted at the bottom
end, with a scale calibrated in specific gravity marked on its side. To test an
electrolyte, draw it into the hydrometer using the suction bulb. Draw enough electrolyte
into the hydrometer to make the float rise. Do not draw in so much electrolyte that the
float rises into the suction bulb. The float will rise to a point determined by the
specific gravity of the electrolyte. If the electrolyte contains a large amount of active
ingredient, its specific gravity will be relatively high. The float will rise higher than
it would if the electrolyte contained only a small amount of active ingredient.

Figure 2-12. - Hydrometer.

To read the hydrometer, hold it in a vertical position and read the
scale at the point that surface of the electrolyte touches the float. Refer to the
manufacturer's technical manual to determine whether or not the battery's specific gravity
is within specifications.

Note: Hydrometers should be flushed with fresh water after each use
to prevent inaccurate readings. Storage battery hydrometers must not be used for any other
purpose.

The routine maintenance of a battery is very simple. Terminals
should be checked periodically for cleanliness and good electrical connection. The battery
case should be inspected for cleanliness and evidence of damage. The level of electrolyte
should be checked and if the electrolyte is low, distilled water should be added to bring
the electrolyte to the proper level. Maintenance procedures for batteries are normally
determined by higher authority and each command will have detailed procedures for battery
care and maintenance.